While the national government remains slow to deal with climate change, many cities have been moving ahead. Why the difference? Well, cities tend to be more homogenous politically, which makes any kind of decisive action easier to push through. But the real reason is that city managers know they will be the first ones forced to deal with the likely consequences of global warming: rising sea levels and flooding, deadly heat waves and water struggles. New York City didn’t just come out last week with the most comprehensive climate-adaptation plan in the world because Mayor Michael Bloomberg is a global-warming believer. The experience of Hurricane Sandy last year — which cost the city some $20 billion — was instructive. Even in the absence of warming, growing population and property values will put major cities on the front lines of extreme weather. Add in climate change, and it could get ugly.

Just ask Los Angeles. The City of Angels has struggled with the basic fact that it is a desert metropolis since its founding. (Just watch Chinatown.) The first three months of 2013 were the driest for California on record, and there’s no relief in sight. Now a new study from the University of California, Los Angeles, suggests that the local mountain snowfall — vital for water supplies — could fall 30% to 40% below 2000 levels by midcentury, thanks to global warming. And if emissions don’t decline and warming is worse than we expect, more snow will vanish, even as greater L.A. continues to grow.

In the business-as-usual scenario — a climate-science term for a model that assumes greenhouse-gas emissions keep growing without any effort to slow them — snowfall levels could fall 42% by midcentury, and over 60% by the end of the century. Here’s lead author Alex Hall of UCLA in a statement:

The mountains won’t receive nearly as much snow as they used to, and the snow they do get will not last as long …We won’t reach the 32ºF threshold for snow as often, so a greater percentage of precipitation will fall as rain instead of snow, particularly at lower elevations. Increased flooding is possible from the more frequent rains, and springtime runoff from melting snowpack will happen sooner.

Obviously this will be a major bummer for Southern California snowboarders, who I guess will just have to take up surfing. But snowfall matters for urban dwells — snowpack in the mountains of California is like a bank for water. It holds the precipitation through the winter, then releases it gradually with the spring melt. But if there’s less snow in the first place, and the spring snowfall occurs earlier and more rapidly because of warming, water supplies become that much more difficult to manage. Add in the fact that Los Angeles is expected to grow to 13 million people by 2050, and you have a management situation, as L.A. Mayor Antonio Villaraigosa said in a statement:

This science is clear and compelling: Los Angeles must begin today to prepare for climate change.

And so it will, just as New York will. But cities can only do so much. They can help their citizens adapt to climate change, but there’s little that one mayor — even one as rich as Mayor Bloomberg — can do by himself to change the way we the country and the world use energy. And it’s energy use that decides climate change — and turns the snow to rain.